BIS(N,N) CHELATED RHENIUM(III) - THE [REL2CL2]CL FAMILY [L = 2-(ARYLAZO)PHYRIDINE]

The reaction of mer-[Re(MeCN)(PPh3)2Cl3] with 2-(arylazo)pyridines (L) affords dark green, diamagnetic, 1 : 1 electrolytes [ReL2Cl2]Cl (aryl of L = Ph,m-MeC6H4 or p-ClC6H4).The ReCl2 moiety is cis (two Re-Cl stretches) and the cation has an overall two-fold symmetry axis (H-1 NMR spectra). The co-ordinated pyridine-nitrogen atoms and azo-nitrogen atoms of each ligand are assigned relative trans and cis geometries respectively. The N=N stretching frequencies are low (1280-1290 cm-1) implying significant Re(t2)-azo(pi*) back bonding. In MeCN solution three nearly reversible redox couples occur with E1/2 values of almost-equal-to 0.9 V [rhenium(IV)-rhenium(III)] and almost-equal-to -0.4, almost-equal-to -0.8 V (azo redox) vs. the saturated calomel electrode (SCE). A metal-to-ligand charge-transfer (m.l.c.t.) absorption is observed near 600 nm. The reduction potentials and m.l.c.t. frequencies are mutually consistent. The remarkable spectral and electrochemical similarities between [ReIIIL2Cl2]Cl and [OsIIL2Cl2] are noted and rationalised.